The present invention relates to the field of multiphase flow measurement. In particular, the invention relates to a method and apparatus for estimating brine water conductivity of multiphase flow mixtures by interpreting mixture permittivity and mixture conductivity measured using a microwave open-ended coaxial reflection probe.
To measure accurately the water fraction and the water-in-liquid ratio (WLR or water-cut) of oilfield oil-water-gas multiphase flows using electromagnetic sensors, it is greatly beneficial to know the brine conductivity which often varies with temperature, salinity and salt species (e.g. due to the water injection). For an electromagnetic water-cut monitor, water conductivity data is conventionally entered manually based on water sample analysis, or measured by flowing single-phase water through the sensing volume (however, the latter is often not possible under production conditions). Some multiphase flow meters utilize dual-energy gamma-ray principle to measure water-cut, with accuracy being highly dependent upon the calibrations of mass attentions of brine water as well as of oil and gas at line conditions. The brine mass attenuation at the lower photon energy is strongly dependent on the salinity and salt species; their departure from the initial calibration values as a water flood proceeds will result in erroneous water-cut and hence oil and water flow rates. Monitoring changes in water salinity on-line, under multiphase flow conditions, is thus highly desirable, especially for permanent (subsea) metering applications.
For characterizing materials in various industrial and scientific applications, open-ended electromagnetic coaxial probes have been used, based on the principle that the measured complex reflection coefficient (ratio of reflected signal to the incident) is dependent on the aperture impedance (thus complex permittivity) of a sample material terminating the probe. For such applications, commercial products are available such as Hewlett-Packard""s 85070C Dielectric Probe Kit (for use with HP series of network analysers). To facilitate complex permittivity inversion, many aperture impedance models of coaxial probes have been developed, such as that disclosed in the U.S. Pat. No. 5,233,306.
The sensitivity depth of an open coaxial probe is shallow, about equal to the inner radius of the outer conductor of the probe (typically only a few millimeters). This means that the probe is sensitive to the bulk electrical property of a material in its close vicinity. In recent years, microwave open coaxial probes have been used in measuring the properties of single-phase and multiphase fluids encountered in oilfields. For example:
1. Complex permittivity of saline solutions (up to about 5 S/m), Hilland J. xe2x80x9cSimple sensor system for measuring the dielectric properties of saline solutionsxe2x80x9d, Meas. Sci. Technol., 8 (1997) 901-10; Nxc3x6rtemann K., Hilland J. and Kaatze U. xe2x80x9cDielectric properties of aqueous NaCl solutions at microwave frequenciesxe2x80x9d, J. Phys. Chem. A, 101 (37) (1997) 6864-6869.
2. Complex permittivity of crude oils and solutions of heavy oil fractions. See, e.g. Friisxc3x8 et al., xe2x80x9cComplex permittivity of crude oils and solutions of heavy crude oil fractionsxe2x80x9d, J. Dispersion Sci. Technol., 19 (1) (1998) 93-126; Tjomsland et al., xe2x80x9cComparison of infrared and impedance spectra of petroleum fractionsxe2x80x9d, Fuel, 75 (3) (1996) 322-332; Folgerxc3x8 et al., xe2x80x9cA broad-band and high-sensitivity dielectric spectroscopy measurement system for quality determination of low-permittivity fluidsxe2x80x9d, Meas. Sci. Technol., 6 (1995) 995-1008; and Folgerxc3x8 K., xe2x80x9cBilinear calibration of coaxial transmission and reflection cells for permittivity measurement of low-loss liquidsxe2x80x9d, Meas. Sci. Technol., 7 (1996) 1260-69.
3. Monitoring density changes in low-permittivity hydrocarbons. See Friisxc3x8 et al., xe2x80x9cMonitoring of density changes in low-permittivity liquids by microwave-permittivity measurements with an open-ended probexe2x80x9d, Meas. Sci. Technol., 8 (1997) 1295-1305.
4. Permittivity measurements of gas hydrate formation in water-oil emulsions. See, Jakobsen et al. xe2x80x9cDielectric measurement of gas hydrate formation in water-in-oil emulsions using open-ended coaxial probesxe2x80x9d, Meas. Sci. Technol.,8 (1997) 1006-15.
5. Permittivity of thin (water/oil) liquid layer backed by gas. See Folgerxc3x8 et al., xe2x80x9cPermittivity measurement of thin liquid film layers using open-ended coaxial probesxe2x80x9d, Meas. Sci. Technol., 7 (1996) 1164-73.
6. The water-cut measuring accuracy by using an open coaxial probe (flush mounted at pipe wall) has been demonstrated to be xc2x15% absolute with gas-cut up to 85%, given the oil and water permittivities (or conductivities). See Skre C D, xe2x80x9cWater-in-liquid probe: system for measuring water-in-liquid ratio at low and high gas volume fractionsxe2x80x9d, Proc. 17th Int. North Sea Flow Measurement Workshop, Clarion Oslo Airport Hotel, Gardermoen, Norway, 25-28 Oct. 1999. Beyond 85% gas-cut, the water-cut is underpredicted due to gas entrainment in the (oil-water) liquid layer close to the probe.
U.S. Pat. No. 5,341,100 discloses the use of a (four-port) coaxial transmission line for the (downhole) continuous measurement of water conductivity and hydrocarbon fraction under dynamic flow conditions. Formation fluid is directed to flow between the inner and the outer conductors of the coaxial structure (about 3xe2x80x3 (7.6 cm) long). The permittivity and conductivity of the formation fluid are inferred from the phase-shift and attenuation of the transverse electromagnetic wave (TEM) measured between the near and far receivers (spaced along the flow tube with respect to a transmitter; a matched load is used to terminate a fourth port farthest from the transmitter). The operating frequency is kept low (about 100 MHz) to avoid large attenuation at high brine salinities and to eliminate measurement ambiguities due to the excessive phase-shift ( greater than 360xc2x0) between the receivers (1xe2x80x3 (2.5 cm) spacing). Water conductivity and hydrocarbon fraction are determined from the fluid permittivity and conductivity; the interpretation scheme is not explicitly disclosed, but could be based on a look-up table approach. Additionally, the system hardware is rather complex.
U.S. Pat. No. 5,675,259 discloses the use of a single or multiple open-coaxial probes (operating at about 1 GHz), arranged almost non-intrusively (aperture flush mounted at pipe/vessel wall) or intrusively (e.g. mounted along the pipe diameter). By rapidly detecting amplitude and phase of the reflected signal, single-phase and multiphase fluids may be identified based on the differences between complex permittvities. Fluid flow rates may be measured by processing signals from probes mounted along a flow pipe. Different depths of investigation may be achieved by varying the intrusion length of the probe inner-conductor exposed to the fluid. However, estimating on-line water conductivity under multiphase conditions is not considered.
U.S. Pat. No. 5,272,444 describes the use of relatively low frequency (tens of MHz) dielectric (impedance) sensor for water-cut and salinity monitoring of water-oil flows, by cross-plotting two sensed parameters such as impedance (ratio) vs. phase, voltage vs. voltage measured at two frequencies. The cross-plot is used as a look-up table which is mapped over a range of water-cut, water resistivity and temperature.
PCT Patent Application No. WO 99/42794 discloses an open-coaxial probe of flat aperture surface (inner conductor not extending into fluid) for flush mount with inner pipe/tank wall (for placement at the liquid rich region even for vertical high-gas annular flows). The depth of investigation can be varied by changing the inner diameter of the outer conductor. To withstand high pressures and temperatures in oilfield tubings, the coaxial probe is designed to have a heat resistant plastic material (of low dielectric constant). To ensure a good seal between the inner and the outer conductors at the probe surface, a conical metal clamp is designed with screw adjustment pressing against a conical insulator (which in turn is pressed against the inner conductor).
Thus, it is an object of the present invention to provide a method of estimating water conductivity in a multiphase mixture of water and other substances comprising the steps of:
obtaining values representing the mixture permittivity and mixture conductivity of the multiphase mixture;
obtaining a relationship between water conductivity and water permittivity; and
estimating water conductivity of the multiphase mixture using at least the values representing mixture permittivity, mixture conductivity, and the relationship between water conductivity and water permittivity.
The invention is also embodied in an apparatus for estimating water conductivity in a multiphase mixture of water and other substances comprising:
one or more sensors for measuring mixture permittivity and mixture conductivity of the multiphase mixture; and
a processor adapted to estimate water conductivity by combining the measured mixture permittivity and mixture conductivity with an obtained relationship between water conductivity and water permittivity.
As used herein the term xe2x80x9cmultiphasexe2x80x9d means mixtures including multiple components such as oil-water, oil-water-gas, as well as water-solids, etc. When the term xe2x80x9cconductivityxe2x80x9d is used herein, it is understood that instead of conductivity one could use equivalents including for example the mathematical reciprocal xe2x80x9cresistivityxe2x80x9d. The term xe2x80x9cpermittivityxe2x80x9d, as used herein, refers to a value relative to a vacuum, and is also commonly referred to as the dielectric constant. The terms xe2x80x9ccomplex permittivityxe2x80x9d as used herein refers to a complex value, with its real part mainly related to xe2x80x9cpermittivityxe2x80x9d and its imaginary part to the permittivity loss (mainly the conductivity loss).